They were originally put into the square hole in the body, then you put some grey putty, I think that's what drip check is used for now, on the pins, as you push the surround on to the body.

The retainer in the pic is broken.
I was looking for these, havent found anyone doing them so far.
Most times lately I've removed surrounds, the cars don't have the retainers on them, just a blob of dried grey putty. So probably don't need (unless you're Jason!)

Cheers
Max